The chemical defense ecology of marine unicellular plankton: Constraints, mechanisms, and impacts

The activities of unicellular microbes dominate the ecology of the marine e nvironment, but the chemical signals that determine behavioral interactions are poorly known. In particular, chemical signals between microbial predat ors and prey contribute to food selection or avoidance and to defense, fact ors that probably affect trophic structure and such large-scale features as algal blooms. Using defense as an example, I consider physical constraints on the transmission of chemical information, and strategies and mechanisms that microbes might use to send chemical signals. Chemical signals in a lo w Re, viscosity-dominated physical environment are transferred by molecular diffusion and laminar advection, and may be perceived at nanomolar levels or lower. Events that occur on small temporal and physical scales in the "n ear-field" of prey are likely to play a role in cell-cell interactions. On the basis of cost-benefit optimization and the need for rapid activation, I suggest that microbial defense system strategies might be highly dynamic. These strategies include compartmented and activated reactions, utilizing b oth pulsed release of dissolved signals and contact-activated signals at th e cell surface. Bioluminescence and extrusome discharge are two visible man ifestations of rapidly activated microbial defenses that may serve as model s for other chemical reactions as yet undetected due to the technical probl ems of measuring transient chemical gradients around single cells. As an ex ample, I detail an algal dimethylsulfoniopropionate (DMSP) cleavage reactio n that appears to deter protozoan feeding and explore it as a possible mode l for a rapidly activated, short-range chemical defense system. Although th e exploration of chemical interactions among planktonic microbes is in its infancy, ecological models from macroorganisms provide useful hints of the complexity likely to be found.